The present invention relates to an input circuit and a control device having the controller. More specifically, the present invention pertains to an input circuit that varies an analog voltage and outputs the voltage to a control system, and to a control device having the input circuit.
FIG. 1 is a circuit diagram showing a prior art control device 100. The control device 100 includes an input circuit 50 and a control circuit 58, and is located, for example, in an air conditioner controller of a vehicle.
The input circuit 50 has a variable resistor 51 and a fixed resistor 57. The variable resistor 51 includes a resistance element 52 and a slider 55.
A first end of the resistance element 52 is connected to a power supply terminal 53, and a second end is connected to a grounding terminal 54. A first end of the slider 55 slides on the resistance element 52, and a second end is connected to an output terminal 56. The output terminal 56 is connected to the control circuit 58 through the fixed resistor 57.
The slider 55 slides between the first end and the second end of the resistance element 52 in accordance with movement of a control knob (not shown). As the slider 55 slides, resistance between the power supply terminal 53 and the output terminal 56 is changed. As a result, the value of the analog voltage outputted from the input circuit 50 to the control circuit 58 is changed.
If the voltage of the power supply is 5V, the analog voltage is varied in a range between zero volts to approximately five volts. When the analog voltage is maximum (approximately five volts) or minimum (zero volts), the control circuit 58 generates a control signal for maximizing or minimizing the heating performance or cooling performance. When the analog voltage is about the intermediate value (2.5V), cooling or heating is stopped. At this time, the slider 55 is approximately at the center of the resistance element 52.
If the grounding terminal 54 of the resistance element 52 is open, the value of the analog voltage is greater than a normal state. If the power supply terminal 53 of the resistance element 52 is open, the analog voltage is always zero. If such abnormalities occur, it may be impossible to set the analog voltage about intermediate level (2.5V). As a result, even if there is an abnormality, cooling or heating cannot be stopped. Contrarily, cooling or heating may be undesirably maximized.
If the output terminal 56 of the variable resistor 51 is open, the analog voltage is not supplied to the control circuit 58 from the power supply. The analog voltage is influenced by external electromagnetic waves and becomes unstable. In other words, the unstable analog voltage is supplied to the control circuit 58. This may cause the control circuit 58 to generate control signals that repetitively and rapidly start and stop cooling and heating.
Accordingly, it is an objective of the present invention to provide an input circuit that detects an abnormality of a voltage generation and easily and reliably informs a control system of the abnormality. Another objective of the present invention is to provide a control device that includes such an input circuit.
To achieve the foregoing and other objectives and in accordance with the purpose of the present invention, an input circuit having a function for detecting an abnormality of the input circuit is provided. The input circuit includes an external output terminal, an analog voltage generator, a first resistor, a second resistor, a third resistor, and a fourth resistor. The analog voltage generator is connected to a power supply and to a ground. The analog voltage generator generates an analog voltage. The analog voltage generator includes a power supply terminal connected to the power supply, a grounding terminal connected to the ground, and an internal output terminal for supplying the analog voltage to the external output terminal. The first resistor is connected to the power supply and to the power supply terminal. The second resistor is connected to the ground and to the grounding terminal. The third resistor is connected to the internal output terminal and to the external output terminal. The fourth resistor is connected to the node of the third resistor and the external output terminal and to the ground. The resistances of the first and second resistors are smaller than the resistances of the third and fourth resistors such that, in a normal state, the analog voltage of the external output terminal is in a normal voltage range, which is narrower than the voltage range between the power supply and the ground, such that, when the grounding terminal is open, the analog voltage of the external output terminal is in an upper limit fail voltage range, which is outside of the normal voltage range, and such that, when the power supply terminal is open, the analog voltage of the external output terminal is in a lower limit fail voltage range, which is outside of the normal voltage range.
Other aspects and advantages of the invention will become apparent from the following description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.